1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a back light unit and an LCD device using the same in which current flow in a plurality of light emitting diodes (LEDs) is uniformly controlled using an optical sensor where a size of a printed circuit board (PCB) is reduced.
2. Discussion of the Related Art
Generally, an LCD device displays desired images on a screen by controlling light transmittance from a back light unit using a liquid crystal panel. The liquid crystal panel includes a plurality of liquid crystal cells having a matrix arrangement and a plurality of control switches switching video signals supplied to the liquid crystal cells.
Recently, back light units have become smaller and lighter in weight. As a result, LEDs have been used instead of fluorescent lamps for back light units. The LEDs have a lower weight, consume less power, and have increased luminance.
FIG. 1 illustrates a related art back light unit having an LED.
Referring to FIG. 1, the related art back light unit 10 includes a plurality of LED arrays 30, a driver 40 for driving the LED arrays 30, and a diffusion plate 20 for diffusing light from the LED arrays 30.
The LED arrays 30, as shown in FIG. 2, include first to Nth LEDs 32 arranged on a PCB 31 in parallel with one another, first to Nth anode electrode lines 34, and first to Nth cathode electrode lines 36.
The PCB 31 is made of either nonmetal or metal. Here, the PCB 31 may be made of metal for improved heat radiation characteristics.
Each of the anode electrode lines 34 is electrically connected to both the driver 40 and anode electrodes of the LEDs 32 thereby supplying a constant current from the driver 40 to the anode electrodes of the LEDs 32. The anode electrode lines 34 are arranged at predetermined intervals in one area of the PCB 31 where the anode electrode lines are parallel with one another.
Each of the cathode electrode lines 36 is electrically connected between cathode electrodes of the LEDs 32 and the driver 40 to supply a ground voltage. The cathode electrode lines 36 are arranged at predetermined intervals in another area of the PCB 31 where the cathode electrode lines are parallel with one another.
Meanwhile, the anode electrode lines 34 and the cathode electrode lines 36 are formed at constant intervals to have a line width which corresponds a current of several hundred mA supplied to the LEDs 32.
Each of the LEDs 32 includes an anode electrode electrically connected to the anode electrode lines 34 and a cathode electrode electrically connected to the cathode electrode lines 36. As shown in the FIG. 2, the LEDs 32 are arranged on the PCB 31 such that they are parallel with one another and repeat the order of red, green and blue.
The LEDs 32 emit light using the constant current supplied from the driver 40 through the anode electrode lines 34 to emit white light through a mixture of red, green and blue lights. As such, white light is irradiated onto the diffusion plate 20. Each of the LEDs 32 arranged on the PCB 31 has a chip type configuration.
The driver 40, as shown in FIG. 3, includes a constant current generator 42 generating constant currents I1 to In, a feedback circuit 44 electrically connected to the cathode electrode lines 36 of the PCB 31 to generate feedback signals Fb1 to Fbn of the PCB 31, and a controller 46 for controlling the constant current generator 42 depending on the feedback signals Fb1 to Fbn from the feedback circuit 44.
The constant current generator 42 generates first to Nth constant currents I1 to In to irradiate each of the LEDs 32. The constant current generator 42 uses an external input voltage Vin controlled by the controller 46. The constant current generator 42 supplies the generated constant currents I1 to In to each of the anode electrode lines 34 of the PCB 31.
The controller 46 increases or decreases the size of each of the constant currents I1 to In supplied to each of the LEDs 32. The controller adjusts the constant current depending on each of the feedback signals Fb1 to Fbn from the feedback circuit 44 to control the constant current generator 42, thereby uniformly maintaining the current flow in the LEDs 32.
The feedback circuit 44, as shown in FIG. 4, includes a plurality of feedback resistors R electrically connected between each of the cathode electrode lines 36 formed on the PCBs 31 and a ground power source. The feedback circuit 44 detects a voltage applied at both ends of each feedback resistor R using the feedback signal Fb and supplies the detected voltage to the controller 46.
The driver 40 generates first to Nth constant currents I1 to In to drive each of the LEDs in parallel. Simultaneously, the driver 40 detects the feedback signal Fb corresponding to the current flow in each of the LEDs 32, which emits light, using the feedback resistors R. The driver 40 supplies the constant current to each of the LEDs 32 even if load characteristics of each LED 32 are varied due to characteristic variation and line resistance of each LED 32.
However, the related art back light unit 10 having the LED fails to control the current flow in each of the LEDs 32 if the voltage of the feedback signal Fb varies as the temperature of the resistor R of the feedback circuit 44 varies.
Furthermore, because the related art back light unit 10 having a LED requires a great amount of current, this complicates the line patterns on the PCB 31 when the size of the PCB 31 increases. Furthermore, the use of metal for the PCB 31 increases fabrication costs.